1. Field of the Invention
The present invention relates to a photothermographic material and a method of forming images of the photothermographic material.
2. Description of the Related Art
In recent years, it has been strongly desired in the field of films for medical imaging to reduce the amount of used processing liquid waste in consideration of environmental protection and space saving. For this reason, technology regarding photothermographic materials as films for medical imaging and for photographic applications, which are capable of efficient exposure with a laser image setter or a laser imager and capable of forming a clear black-toned image with high resolution and high sharpness is desired. Such photothermographic materials can eliminate use of liquid processing chemicals and can provide users with a thermal development system which is simpler and does not contaminate the environment.
Although similar requirements also exist in the field of general image forming materials, an image for medical imaging requires a particularly high image quality excellent in sharpness and granularity because a delicate image representation is necessitated. Also an image of blue-black tone is preferred in consideration of easy diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as ink jet printers and electrophotographic systems, are available as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing an organic silver salt are described widely. Particularly, a photothermographic material generally has an image-forming layer that includes a catalytically active amount of a photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and, optionally, a color toning agent for controlling the tone of silver dispersed in a binder matrix. After imagewise exposure, the photothermographic material is heated to a high temperature (for example, 80° C. or higher) and black silver images are formed by an oxidation/reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizer) and a reducing agent. The oxidation/reduction reaction is promoted by the catalytic effect of latent images in the silver halide formed by exposure. As a result, black silver images are formed in an exposed area. The Fuji Medical Dry Imager FM-DPL has been commercially available as a medical image forming system using photothermographic materials.
Since the ingredient componetns described above are contained in a photothermograhpic material and all of the components remain even after development, there are many issues concerning storage stability. Often studied methodologies for solving the problems have so far included change of the composition contained in the image-forming layer and addition of new compounds. Various methods have been studied and successful results have been achieved such as, for example, improvement of the print out property by changing silver halides to those of high silver iodide content as described in JP-A No. 8-297345 and JP No. 2785129, suppression of the occurrence of fogging by the addition of polyhalogen compounds as described in JP-A No. 2001-312027, or increase of the silver behenate content in a non-photosensitive organic silver salt as described in JP-A No. 2000-7683.
Since the image-forming layer is a portion directly forming images, it is extremely important to study the compositions contained in the image-forming layer as a method of improving the storage stability. However, since such compositions are present in admixture in the image-forming layer, sensitivity tends to be lowered when an attempt is made to improve the storage stability, and image density tends to be lowered when an attempt is made to suppress occurrence of fogging. It is extremely difficult to simultaneously attain both the reciprocal properties of storage stability and higher sensitivity, and those of suppression of fogging and image density.
As described above, a photothermographic material is prepared in a well-balanced manner so as to benefit from the advantages of the respective compositions to the utmost degree, improvement of storage stability being difficult with a mere change or addition of one composition. There is always a keen desire for a method of improving storage stability without offsetting the features of individual compositions.